Talk:Uranium-thorium dating

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I though it was measuring radioactive decay of uranium isotopes 238U and 235U?

Not at all. 235U is commonly used in U-Pb dating (as it ultimately decays to 207Pb) and rarely in 235U-231Pa dating. 231Pa is an intermediate decay product in the 235 decay series much as 230Th is an intermediate decay product in the 238U decay series. U-Pa dating is sometimes used as a check on U-Th dating (as any post-depositional alteration of a sample would affect the results of the two techniques differently) but not very often as it is analytically more difficult and has a much lower maximum age range than U-Th. Actinide 14:16, 12 February 2006 (UTC)

[edit] Age limit POV

The age limit is based on a uniformitarian assumption; however, this assumption is not indicated. A reader would currently be mislead into believing that the applicability of this dating method to look 500,000 years into Earth's past is noncontroversial.

There is an argument that under POV equal weight, only the viewpoints of scientists should be considered in determining whether the viewpoint of nonuniformitarianism is significant. Under WP:SPOV, I could see this (I think - haven't looked into SPOV much), but WP:NPOV's undue weight section does not call for exclusion of a class of people (in this case non-scientists) when determining whether a viewpoint is held by a significant minority. Perhaps the undue weight section should be clarified, but excluding the viewpoint of the vast majority of the population seems quite contrary to the spirit of NPOV and Wikipedia in general.

This is particularly true in this case because scientists typically do not investigate whether uniformitarianism or nonuniformitarianism is correct; they just assume uniformitarianism. This puts their viewpoints in no stronger position than those of the population at large.

Thoughts on qualifying the assumption?

--Ed Brey 11:41, 21 August 2006 (UTC)

If you have specific concerns with U-Th dating then by all means raise them. But what you inserted into the third paragraph sits uncomfortably there and has no relevance to the content of that paragraph, i.e. the unusual nature of U-Th disequilibrium dating that limits its age range to only 500,000 years vs the billions of years for U-Pb (i.e. a different use of the same decay chain).

If what you are tryng to say is that possible limitations to the technique have not been adequately discussed then thats fine - this article as it stands is little more than a stub. By all means add a section on underlying assumptions (althogh uniformitarianism doesn't sit too well as having much to do with it there either unless you are extending it to the idea that radioactive decay rates are variable or the chemical properties of Th232 and Th230 were somehow different to each other in the past). As a starting point you could try something lke this:

U-Th dating, like any geochronological technique depends on underlying assumptions, in this case two of them. Firstly, there must have been very little Th230 in a given sample at its time of formation, and secondly that sample must have been closed with respect to all elements in the 238U decay chain over its lifetime. The first assumption is fortunately very easy to test in that Th has another naturally-occuring isotope, Th232 which is about 180,000 times more abundant than the 230Th we are trying to measure. So if Th was for some reason not excluded from a sample at its time of formation then we expect to see gretly elevated levels of Th232, and as a rule of thumb age determinations are usually discarded as unreliable if the ratio of 232Th to 230Th in the sample concerned is found to be greater than roughly 2,000.

The second assumption is difficult to test in individual samples beyond a close invesitigation of the physical and chemical composition of each sample, but fortunately we can show that where open-system behavior is present it must have the effect of scrambling the apparent ages of different parts of any given object or horizon. This means multiple age determinations on the same object (e.g. of different parts of one fossil coral) will give different answers if open system processes have been active, rather than all being in agreement as is expected for a closed system. On this basis, some materials such as teeth, bone and molluscs have been found to be consistently unsuitable for U-Th dating whereas fossil coral reefs are moderately reliable and speleothems have been found to be extraordinarily reliable.

Actinide 12:35, 23 August 2006 (UTC)